That carbon dioxide is like a growth hormone for ragweed. It reacts by pumping out pollen like it's on steroids: up to 55 percent more pollen per plant. And that is an allergy sufferer's worst nightmare.

To make matters worse, the warmer weather also extends the growing season, meaning plants have more time to spread allergic woes.

If you live in the city, there's a third problem: all that concrete, baking in the sun, raises temperatures and carbon dioxide levels even more.

It's not just ragweed that's getting a kick out of all the carbon dioxide, according to Harvard researchers. A far more poisonous form of poison ivy may also begin to flourish.